The use of mirror-lined ducts for transporting visible light to provide general illumination, e.g. in a building or office environment, is known. Depending on the details of construction, such “light ducts” may provide: a large cross-sectional area and large numerical aperture, enabling larger fluxes with less concentration; a robust and clear propagation medium (i.e., air), for lower attenuation and longer lifetimes; and a potentially lower weight per unit of light flux transported, in comparison to fibers or solid light guides. Light ducts can also be used with remote light sources, such as the sun or known electrically powered light sources, to transport remotely generated light to an interior environment.
In some cases, the light duct may include a section that partially transmits and partially reflects light propagating in the duct. The partial transmission is to extract some of the light from the duct to provide exterior illumination. The partial reflection is to allow the remainder of the light in the duct to continue propagating to more distal portions of the duct. In some cases, the partially reflective/transmissive section utilizes a film stack that includes a perforated mirror film and a structured optical film.